The present invention is related to a method and apparatus for feeding chemical into a liquid flow. The method and apparatus of the invention are particularly well applicable to feeding of very small chemical volumes in precise amounts into large process liquid flows.
Naturally, there is practically an innumerable amount of prior art methods of feeding various chemicals into liquid flows. However, these methods may be divided into a few main categories as can be seen from the following. Firstly, it is quite possible to just let the liquid to be added flow freely into a second liquid without employing any special regulation or mixing means. This method of adding cannot be employed in situations where the mixing ratio or the uniformity of the mixing is important. Neither can it be employed in situations where the price of the chemical to be added is of significance.
The next applicable method is to feed the chemical in a precise ratio to the liquid flow, whereby correct and economical dosage is obtained. However, even in this case one has to take into account that usually the dosage of the chemical is slightly excessive compared to the optimal dosage, because the mixing is known to be inadequate. The mixing may be improved, though, by feeding the chemical, e. g., through a perforated wall of a flow channel, whereby at least the chemical to be mixed may be spread throughout the entire liquid flow. As the last example, a situation may be discussed, where the chemical is fed in a precise proportion either into the liquid flow upstream of the mixer or through the mixer itself. In that case, the efficiency of the mixing of the chemical into the liquid flow is totally dependent on the mixer design.
Finnish patent no. 108802 discusses as an essential case of mixing related to paper manufacture the mixing of a retention aid into fiber suspension flowing to the head box of a paper machine. In paper manufacture, retention chemicals are used especially in order to improve the retention of fines at the wire section of a paper machine. In the Finnish patent mentioned the mixing device is in fact a conical nozzle with an inlet for the retention chemical. The mixing device is functioning and efficient both in the mixing of retention chemicals and other chemicals in the short circulation of a paper machine and also in other applications in the paper and pulp industry. However, it has been noticed in connection with some applications that various solid substances carried with the feed and/or dilution liquid tend to accumulate in the device. In other words solid material tends accumulate in the device parts converging in the flow direction, which gradually harms the flow profile, the flow itself and in the end tends to clog the device. FI patent application no. 20021350 describes a self-cleaning chemical feed nozzle. In other words when the nozzle starts to become clogged a change take place in its flow conditions which causes a reaction of the nozzle to open wider the cross-sectional flow area of the flow channel in which the solid material in question flows with the fiber suspension; as a result of this the solid particles attached to the channel can get loose from the nozzle and flow on.
In this kind of applications, i. e. feeding for example retention chemicals into a fiber suspension, the mixing devices and the nozzles described in the publications mentioned work well but in cases where only very small amounts of chemicals are needed in relation to the suspension flow to be fed, the operation of the these nozzles is not the best possible for example because they cannot guarantee an adequately homogenous mixing of the chemical into the process liquid flow because of the small volume of the chemical.